This invention relates generally to telemetering systems of the multiplexing type adapted to transmit data derived from a group of sources to a remote receiving terminal, and more particularly to a low-cost and efficient system for transmitting process control data.
In industrial process control, it is necessary to transmit data obtained at various points or stations in the process to a remote indicating or control terminal. The data to be conveyed from the field stations to the receiving terminal may be changes in pressure, temperature, flow rate or any other process variable. In most cases, this data is derived by means of analog sensors which convert the sensed process variables into corresponding electrical analog signals.
A telemetering system in which the output of each sensor is fed to the remote terminal over a separate wire line is not feasible when the distance between the sensors and the terminal is long and many sensors are involved. The large number of lines entained by the telemetering system and their length may make the system prohibitively expensive.
Multiplexing techniques are known which act to sequentially transmit digital values derived from continuous or analog data to a remote terminal over a single main channel, thereby avoiding the need for as many telemetering lines as there are sensors. Multiplex systems for analog information usually use frequency division to transmit this information, the several input signals being each modulated onto a subcarrier and the combined for transmission.
Also in use are time-division multiplexing systems which employ a commutator (electronic or mechanical) at the transmitting station to sample each data source in sequence. The samples transmitted over the main channel are separated at the remote terminal into appropriate sub-channels by means of a similar commutator which runs in synchronism with the transmitter commutator. Existing time-division multiplexing systems capable of operating reliably to transmit signals in their proper sequence and without overlap are typically of the digital type and are relatively complex and expensive. They add substantially to construction and operating costs in process control installations.
In a typical digital multiplexing system, the transmitter includes a transmitter commutator, an address generator, an analog-to-digital converter and a transmission controller. Each input signal selected by the commutator under the control of the address generator is converted by the code converted to a binary data signal. The address and data signals are combined into one digital word to which is added some error detection bits by the code converter. These combined signals are sent out by the transmission controller to a common channel in a serial manner. For this purpose a wide channel bandwidth is required, making the common channel expensive to install. Moreover the broad bandwidth common channel has a high degree of noise-sensitivity.
At the receiver, the serial data is received by an input register which converts the serial data to its parallel format for further processing. Besides the input register, the receiver includes an address register, a digital-to-analog converter, an error detector, a verification circuit and a receiving commutator. When data is received, the error detector checks to see if any error has occurred in the course of data transmission. If no error is detected, the verification circuit sends back a verification signal to the transmitter. Should an error be detected, then the verification circuit requests the transmitter to again send the same data. The address component of the received data is fed to the address register which controls the receiver commutator, the data component being fed to the digital-to-analog converted to change back into analog form.